1. Field of the Invention
This invention relates to a laminated metal structure, and in particular to a metal laminate having an etch stop to prevent undercutting of the adhesive of the laminated structure.
2. Description Relative to the Prior Art
Mechanical processing of patterned thick laminated metal sheets is a viable approach in fabricating devices such as resistive ribbon and thermal print heads when the dimensions of the patterned device is on the order of mils. Referring to FIG. 1, the method involves laminating a thick metal sheet (10) on the order of 1 to several mils thick onto a carrier substrate (12) such as alumina (Al.sub.2 O.sub.3) or Kapton, a pdyimide based plastic, and then patterning the metal sheet by photolithographic or other means. The metal sheet is conventionally laminated using a variety of organic adhesives (14) compatible with the substrate. Cutting means such as grinding, gouging or sawing then allows the laminated structure to be diced out along the patterned outline to obtain the patterned part without fear of delaminating the structure. However, as the resolution of the patterned device features is increased into the micron range, such mechanical processing becomes impossible.
For high resolution patterns, alternate processing means have been explored with vacuum processing the method of choice. Vacuum etching of the metal sheet is an attractive method of metal removal to leave the patterned part adherent to the substrate. The most prominent of these methods is Reactive Ion Etching (RIE) which is based on glow discharge plasma techniques.
The vacuum method focuses on masking or photomasking a pattern on the metal sheet with subsequent gas phase chemical etching to remove the exposed metal. The RIE process is tailored to etch the specific metal of choice by adjusting the chemical system and/or the RIE etching conditions. The majority of chemical systems rely on generating reactive species such as fluorine (F), chlorine (Cl), and/or oxygen (O) in the form of ions, radicals and neutrals. These species are present individually, in combination or conjunction with other chemical fragments within a glow discharge. These species can be generated by using reactant gases such as perfluoromethane (CF.sub.4), perfluoroethane (C.sub.2 F.sub.6), carbon tetrachloride (CCl.sub.4), chlorine (Cl.sub.2), oxygen (O.sub.2), boron trichloride (BCl.sub.3), and sulfur hexafluoride (SF.sub.6).
The majority of RIE processes are designed to provide an etch stop. The etch stop provides an end point of thee etching process by providing a surface that can not be etched by reactive species or a surface where the etch rate of the surface is several orders of magnitude smaller than the selective material being etched.
The critical problem that exists with RIE methods when used with laminated structures is related to the compatibility between the laminate adhesive and the reactive species generated in the RIE process. The laminate adhesive system must have the following properties: a) be impervious to chemical breakdown in the plasma, b) the adhesive can not interfere with the selected metal etch process, and c) the adhesive must survive the local temperature effects during the etching process. The reactive species generated by the reactants listed above, chemically react with and continuously degrade adhesives containing Organics. The degradation products subsequently interfere with the RIE etching process of the metal layer by generating unfavorable polymeric by-products that redeposit on the surface of the metal layer or alter the gas phase chemistry in a manner that the etch selectivity is not in favor of the metal layer. The majority of such adhesives can be readily attacked by fluorine, chlorine and oxygen containing species with the activated plasma. RIE attack of the laminate adhesive results in dramatic undercutting and severe delamination of the metal from the surface of the substrate. Referring to FIG. 2, patterned metal parts 16, RIE processed from a metal sheet (10, FIG. 1) adhere to a substrate 12 by means of adhesive 14. It will be noted that the adhesive 14 has been undercut at the patterned pieces 16, and that because of the undercutting the metal pieces 16 are severely delaminated from the substrate 12.
The present invention relates to a laminate suitable for RIE etching using an adhesive containing organics without the problem of delamination due to destruction of the adhesive, and a method for making and using such a laminate.